Opal glasses are well known and have been produced for many years. Such glasses are widely used in decorative consumer products such as "Tiffany" windows, jewelry, and blown glass ornaments and utensils. The opacity of opal glass is the result of a glass structure which includes two phases of different refractive indexes. In most cases a light diffusing phase is dispersed throughout a transparent glassy phase. Glass compositions which form a second, light diffusing phase automatically upon cooling from a molten state are known as spontaneously opacifiable glasses. Due to the relative simplicity of producing spontaneously opacifiable glasses, they are preferred to opal glasses which require a subsequent heat treatment to produce the light diffusing phase.
Cullett from recycled glass such as container glass or window glass is becoming an increasing popular and available raw material for the production of glass products. Container glass cullett, however, is approximately 13% alkali metal oxide in the form of Na.sub.2 O and 12% alkaline earth oxide in the form of CaO. These oxide levels are very near to the acceptable limit for alkali metal oxides and alkaline earth oxides in opalized glass. Attempts to add conventional opacifying agents, such as cryolite (Na.sub.3 AlF.sub.6) or fluorspar (CaF.sub.2), to form opal glass from cullett have failed because these opalizing agents contain substantial amounts of either calcium or sodium. Thus, a glass making batch which contains both an effective amount of a single traditional opacifying agent and a high percentage of cullett produces a glass composition which contains excessive amounts of sodium oxide or calcium oxide. Mixtures of cryolite and fluorspar are not suitable because they form an immiscible phase which excessively stratifies in the melting process.
Previously, to form opal glass of suitable quality from cullett it has been necessary to add large amounts of silicon dioxide and other basic glass making ingredients so as to reduce the calcium and sodium content of the batch. Adding large amounts of raw materials, however, greatly reduces the advantages of using recycled glass.